1. Field of the Invention
The present invention relates to a signal processing apparatus which processes a signal from an optical signal output device for detecting a displacement of a displacement detection target. Further, the present invention relates to an optical displacement detection system which includes the signal processing apparatus and detects a displacement of a displacement detection target.
2. Description of the Related Art
The following technique is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 48-78959 as a technique related to an optical displacement detection system, which detects a displacement of a displacement detection target. The foregoing Publication No. 48-78959 discloses a photoelectric detector. The photoelectric detector has a structure in which a slit or reflection plane is provided in line with a predetermined space with respect to a detection object, and detects an optical pulse resulting in the slit or reflection plane. Moreover, the photoelectric detector is configured so that the radial length of the slit or reflection plane becomes gradually large clockwise.
FIG. 14 is a side view showing an optical signal output device of the photoelectric detector (i.e., optical displacement detection system) disclosed in the foregoing Publication No. 48-78959. As seen from FIG. 14, a light beam B emitted from a light source 1 is irradiated on a slit of a detection object which is a scale 4, and then, the transmitted light is detected by a photodetector 2.
FIG. 15 is a top plan view showing the scale 4 in the photoelectric detector disclosed in the foregoing Publication No. 48-78959. As shown in FIG. 15, the radial length of the slit formed on the scale is increased or decreased with a predetermined space based on a reference position A or B as a reference with respect to the rotational direction shown by the arrow of FIG. 15 (FIG. 15 shows an example of increasing the radial length).
FIG. 16 is a graph showing an output signal of a photodetector in the photoelectric detector disclosed in the foregoing Publication No. 48-78959. In the graph shown in FIG. 16, the horizontal axis takes a displacement (rotational angle) of a detection object, and the vertical axis takes an output of the photodetector 2. When the scale 4 is rotated counterclockwise, the aperture length of the slit is decreased. In this case, as seen from FIG. 16, the following characteristic appears; specifically, the amplitude of the output signal from the photodetector gradually decreases. The output signal having the foregoing characteristic is suitably operated using a signal processing apparatus, and thereby, it is possible to detect a rotational speed of a detection object. Moreover, a change of the amplitude is detected, and thereby, it is possible to detect a rotating direction of the scale 4.
The foregoing Publication No. 48-78959 does not have the following description. Specifically, according to the foregoing structure, the magnitude of an amplitude or direct current (DC) component of an output signal with respect to a rotating displacement is previously checked. Then, the amplitude or DC level of the output signal is measured, and thereby, it is possible to detect an absolute position from a reference position of a rotational angle of the scale 4. In this way, the photoelectric detector disclosed in the Publication No. 48-78959 is capable of detecting a kinematic direction and an absolute position of a detection object based on the amplitude of a detection signal, which changes with the movement of a detection object.
Here, in the technique disclosed in the Publication No. 48-78959, the output signal from the photodetector 2 has a DC component, which is affected considerably by an optical arrangement of the light source 1 and the scale 4, an output change of the light source and defect of the scale 4. In order to improve a detection sensitivity of an absolute position or to widen a detection range of the absolute position, the aperture length of the slit is set smaller. In this case, the smaller the minimum value of the aperture length of the slit is set, the more the amplitude of the output signal is reduced. Therefore, a noise component becomes relatively large in the output signal from the photodetector 2. As a result, the detection performance (resolving power, stability) is reduced at a portion where the aperture length of the slit is small.
Moreover, if the absolute position is detected using the configuration disclosed in the Publication No. 48-78959, there is a need to previously check an amplitude characteristic of an output signal from the photodetector 2 with respect to a rotating displacement. In this case, the amplitude characteristic of an output signal with respect to a rotating displacement changes resulting from environment, attaching shakiness of a sensor, an age-based change and defect on a scale, for example. For this reason, the detection accuracy and reliability are extremely low.